Conventional electric power supply systems mounted on hybrid electric vehicles (HEV) and electric vehicles (EV) use electric power of a high-voltage battery as power for operating auxiliary equipment by stepping down the voltage of the electric power using a DC-DC converter and accumulating the power in a lead battery (for example, see Japanese Laid-open Patent Publication No. 2014-231324).
The European Union (EU) adopted the directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, or, the restriction of hazardous substances directive (RoHS). The directive designates several kinds of substances including lead as hazardous substances and restricts their use.
Furthermore, it is likely that the End-of Life Vehicles Directive (the ELV directive), which is an environmental policy addressing end-of-life vehicles, restricts use of lead batteries for new model vehicles, after January, 2022.
A lithium-ion rechargeable battery (LiB) as replacement of a lead battery has therefore been studied for use as an auxiliary battery in the above electric power supply system.
Replacing a lead battery with an LiB for an auxiliary battery, however, has issues of the accuracy and the quality of electric power supply.
For example, use of an LiB needs processes such as calculating the state of charge (SOC) of the LiB based on the open circuit voltage (OCV) and equalizing the energy capacity between cells connected in series in a battery pack of the LiB, with the LiB disconnected from the electric power supply system.
The LiB used as an auxiliary battery has to be continuously connected to the supply system from the DC-DC converter because of the necessity of dark current. In this circumstance, neither calculation of the SOC nor equalization in the amount of charge between cells can be conducted, which problematically causes, for example, accumulation of errors of the SOC and variation of the energy capacity between cells.